Metal-air batteries have long been known, and comprise one or usually a plurality of air-depolarizable cells connected in series, parallel, or series/parallel so as to provide the required battery terminal voltage with the requisite current capacity. Most often the metal used in the metal-air cells of metal-air batteries is zinc, but cadmium, iron or other metals may also be used.
In any event, metal-air cells have a higher energy density than the usual "dry" cells or other primary cells such as zinc/manganese dioxide or zinc/mercuric oxide cells; and in the most common instance, zinc-air cells may have three times or more the energy density of such other cells. There is, therefore, a great advantage in using such cells in multicell batteries, but to do so requires that the battery be vented or otherwise provide for access to the atmosphere-- or, more correctly, atmospheric or gas-communication access to the cells-- so as to provide for the admission of atmospheric oxygen which is required for depolarization of the cells and consequent electrochemical provision of electrical power from the battery.
Each cell has one and usually a plurality of openings in the cell container to facilitate oxygen flow to the gas-depolarizable electrode of the cell, and therefore the battery case in which a plurality of cells are placed must also have at least one opening of sufficient size to permit the requisite gas flow during operation of the battery. However, if the battery case remains constantly open to the atmosphere so that there is constant gas communication from the metal-air cells within the battery case to the ambient in which the battery is operating, certain difficulties may occur under certain conditions. For example, where the ambient in which the battery case is placed has, at any time, a high relative humidity, and the battery case is open even though the battery is not working, there may be excessive water vapour influx into the metal-air cells, causing swelling or other damage within the cells, and thereby rendering the battery inoperative. Likewise, in conditions of very low relative humidity of the ambient, when the battery is not otherwise operating but there is gas communication to the cells, there may be water vapour outflow from the cells, causing drying up of the cells with the possibility of internal short circuits occurring or other damage which, again, may render the battery inoperative.
In all events, there is also the possibility when an intermittently operating metal-air battery is constantly exposed to the atmosphere, of some carbon dioxide absorption by the alkaline cell electrolyte of each of the cells, thereby decreasing the energy capacity of the cells.
In all of the circumstances referred to above, the contemplation is made of intermittent operation of the battery rather than steady state operation, and therefore it is considered that the intermittently operating battery is required to have a relatively long operative life during which it may be intermittently operated. The life of the battery may be considerably enhanced, therefore, by providing a vent and a vent cover for the battery, where the vent would be opened at the time that the battery is put into operation and closed each time the battery operation ceases. This can be manually accommodated, but requires manual operation of a valve or other device functioning as the vent cover, and is either left to the memory of the user or requires rigorous procedure for putting the battery into and out of service. This may not always be possible, however, where the battery is connected to an intermittently operating load in a remote location, such as signal lights, beacon generators and the like.
One approach to overcome certain of the difficulties referred to above is shown in Derksen U.S. Pat. No. 2,468,430 dated Apr. 26, 1949, where an oxygen inlet is provided with a sealing mechanism for a metal-air battery, but where manipulation by the user of the battery is required. In at least one embodiment contemplated by that patent, for a pocket lamp, a vent is associated with a push-button switch by which operation of the flashlight is initiated.
The present invention overcomes all of the difficulties referred to above, and provides for automatic opening and closing of a vent to permit gas communication of the cells within the battery case to the ambient only during the period when the battery is in operation. This is generally accommodated by the provision of a vent and a vent cover for the battery case, where the vent cover permits gas communication to the interior of the battery case when it is open, and is associated with an electrically operated actuator means so as to be movable upon the application of electrical power to the actuator means in such a manner that the vent may be opened. A source of electrical power for the electrically operated actuator means is provided--and usually that source is the battery itself; as well as means for initiating the application of electrical power to the actuator means at the time when the battery is placed in operation.
It is recognized that, even where the source of electrical power for the electrically powered actuator by which the vent cover is opened may be the battery itself, there is sufficient residual oxygen contained within the battery case even after it has been closed or sealed following the last operation to provide for the initial load requirements which may be placed on the battery as well as to provide power for the electrical actuator. In some circumstances, however, it may be desirable that the source of electrical power for the actuator may be an auxiliary primary cell or battery, such as a sealed dry cell contained within the battery case or otherwise.
The nature of the actuator is generally an electrically heated element which may either be associated with a bimetal element which functions as the vent cover element; or the electrical actuator may be associated with an electrical heater in such a manner that it changes its physical configuration upon heating, and is associated with a flexible element having elastic memory which is normally biased to the vent-closed position but which may be forced to the vent-opened position upon heating of the actuator element.
One of the preferred vent cover elements, discussed hereafter, is an electrothermal actuator which is a bimetal element having a heater element physically secured thereto in such a manner that the energy requirements to cause sufficient heating of the bimetal element that it changes its configuration may be less. However, in such an element, there would be a requirement for at least a certain continuous supply of power to maintain the heating effect on the bimetal element so as to keep it in the requisite vent-opened position. Other embodiments may also be contemplated, as discussed hereafter, including a bimetal element associated with two heater elements so that momentary application of electrical power to one heater element opens the vent and momentary application of electrical power to the other heater element closes the vent. Such an arrangement, of course, requires a more complicated actuator control circuit, however.